1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include numerous base stations (also referred to as node-Bs) that may be used to provide wireless connectivity to mobile units. When a user of the mobile unit wants to place a call or when the network detects a call destined for the mobile unit, the base station and the mobile unit form a wireless communication link over the air interface between the devices. Information may then be exchanged between the base station and the mobile unit. Mobile units typically encounter a heterogeneous environment including end points and inter-network interfaces that operate according to different standards and protocols. For example, the wireless communication system may include end points and/or inter-network interfaces that operate according to Third Generation Partnership Project (3GPP) standards and/or protocols for Universal Mobile Telecommunication Service (UMTS) Terrestrial Radio Access Network (UTRAN) communication, e.g., the Long Term Evolution (LTE) standards. The wireless communication system may also include other end points and/or inter-network interfaces that operate according to other standards such as the Third Generation Partnership Project 2 (3GPP2) standards and/or protocols for high rate packet data (HRPD) communication. Furthermore, a wireless communication system is often required to interwork with end points and interfaces that operate according to IETF or other protocols. Mobile units therefore often support multiple types of interfaces to allow them to establish links with different types of networks and to hand off between these networks.
Information transmitted over the air interfaces may be encoded as digital information, i.e., bits. One increasingly popular technique for transmitting voice information associated with a call is the Voice-over-Internet-Protocol (VoIP). In VoIP, a digital representation of the voice information is transmitted as a payload in an IP packet. For example, mobile units such as cell phones include vocoders that can convert a received analog signal representative of audio input into a digital representation that can be transmitted over the air interface. Vocoders use knowledge of the patterns in human speech, as well as information derived from the audio signal itself, to efficiently encode voice signals. The encoded digital representation may then be transported as the payload in one or more IP packets. Vocoders are also used to convert received digital payloads into an analog signal that corresponds to the audio input used to create the digital payload. Examples of vocoders are adaptive multi-rate (AMR) vocoders and extended variable-rate coding (EVRC) vocoders.
In a heterogeneous environment, the numerous mobile units in the wireless communication system may not all be using compatible vocoder technologies. For example, mobile units may include AMR vocoders, EVRC vocoders, or (in the case of dual-mode mobile units) both AMR and EVRC vocoders. Various network interfaces may also use different vocoder technologies. LTE network interfaces typically support AMR vocoders, whereas the interfaces in HRPD networks typically support EVRC vocoders. Different types of vocoders can utilize incompatible digital formats and consequently one type of vocoder cannot reproduce the audio input represented by digital information generated by a different type of vocoder. Digital service providers may therefore implement transcoders to transcode the digital information formed using one type of vocoder into a format that is compatible with a different type of vocoder. Adding a transcoder into the communication path between two mobile units may permit the mobile units to exchange VoIP packets even though the mobile units use incompatible vocoders. For example, a transcoder may be used to convert packets formed by an AMR vocoder into a format that can be understood by an EVRC vocoder.
A transcoder can be inserted into the communication path when a call is initiated between mobile units that use incompatible vocoders. For example, a network entity may be used to determine the type of vocoder supported by each user when a first user initiates a call to a second user. No transcoder is inserted into the communication path if the two users implement the same vocoder (or compatible vocoders). However, the transcoding requirements of each mobile unit can change when the call is handed over. For example, a call that is initially established between to mobile units that implement AMR vocoders over a network that supports AMR vocoders may not need any transcoding. But if one of the mobile units hands-off to a network or network interface that requires a different vocoder, such as an EVRC vocoder, then transcoding between the two vocoder technologies may be necessary.
One potential solution to this problem is to allocate a transcoder to every call when the call is initiated. The transcoder will then be available to perform transcoding if one of the mobile units hands-off to a network interface that requires an incompatible vocoding technology. However, allocation of the transcoder when the call is initiated and the subsequent operation of the transcoder consume hardware resources and processing time. If no transcoding is necessary, or if transcoding is only necessary during a relatively small fraction of the call duration, then the hardware resources and processing time devoted to supporting the transcoder are largely wasted. Furthermore, transcoding requires decoding the received information according to the first vocoder technology, “listening” to the decoded audio signal for a short time, and then re-encoding the audio signal according to the second vocoder technology using information derived while listening to the decoded audio signal. This process may result in radio bearer delays which may degrade the user experience.
Another potential solution to this problem is to allocate a transcoder after the network receives a handoff request from a mobile unit and determines that the mobile unit is handing off to a network interface that requires an incompatible vocoder technology. Conventional wireless communication systems require that any delays created by a handoff be limited to avoid degrading the user experience. For example, some standards require that switching delays last no more than approximately 300 ms. However, the potential delays that can result when a mobile phone roams between network types may be significantly longer than this limit because the network must determine if transcoding is necessary in the new network. For example, if a first mobile unit roams from a UTRAN network to a HRPD network it must switch from the AMR vocoder to the EVRC vocoder. If the first dual-mode mobile unit is in a call with a second mobile unit that only implements an AMR vocoder, then the network must insert a transcoder into the communication pathway from the first mobile unit to the second mobile unit. The time required to detect the need for the transcoder and then insert the transcoder into the pathway is significantly longer than the 300 ms limit set by conventional wireless communication systems.